To generate electrical power by means of fuel cells a larger number of fuel cells are ordinarily arranged in the form of a stack, each fuel cell having an anode, a cathode and an electrolyte arranged in between. The individual fuel cells are each separated by bipolar plates and electrically contacted. Current collectors are provided on the anodes and cathodes, which serve for electrical contact of the anodes and cathodes, on the one hand, and to supply reaction gases to them, on the other. Sealing elements are provided in the edge area of the anode, cathode and electrolyte matrix, which form lateral sealing of the fuel cells and therefore the fuel cell stack from emergence of anode and cathode gas.
The electrolyte material in a molten carbonate fuel cell typically consists of binary or ternary alkali carbonate melts (for example, mixed melts of lithium and potassium carbonate), which are fixed in a porous matrix. Molten carbonate fuel cells typically reach working temperatures of about 650° C. during operation. A reaction of hydrogen with carbonate anions to water and carbon dioxide with release of electrons then occurs on the anode side. Oxygen reacts with carbon dioxide to carbonate ions on the cathode side with absorption of electrons. Heat is then released. The alkali carbonate melts used as electrolyte, on the one hand, supply the carbonate ions necessary for the anode half-reaction and, on the other hand absorb carbonate ions that form in the cathode half-reaction. A hydrocarbon-containing energy carrier, like methane, for example, which can come from natural gas or biogas, as well as water, are generally supplied in practice to the anode side of the fuel cell, from which hydrogen required for the anode half-reaction is produced by internal reforming. The anode waste gas is mixed with additionally supplied air and then oxidized catalytically to eliminate any residual components of the fuel gas. The formed gas mixture now contains carbon dioxide and oxygen, i.e., precisely the gases required for the cathode half-reaction so that anode waste gas can be introduced directly to the cathode half-cell after fresh air supply and catalytic oxidation.
The hot exhaust emerging at the cathode output is pollutant-free and can be further used for heat. The electrical efficiency of the molten carbonate fuel cell is already 45 to 50% and when the heat released in the overall process is used, an overall efficiency of about 90% can be achieved.
The known fuel cell arranged by the applicant is described in detail, for example, in the international patent applications WO 96/02951 A1 and WO 96/20506 A1 and in German patent application DE 195 48 297 A1, incorporated herein in their entirety.
The essential components of the known fuel cell arrangement are schematically depicted in FIGS. 1 and 2 in a frontal and lateral cross-sectional view. The fuel cell arrangement designated overall with reference number 10 has a horizontally lying fuel cell stack 11, i.e., consisting of vertically arranged, plate-like elements, which is arranged in a heat-insulated, gas-tight protective housing 12. Fuel gas is supplied via a fuel gas line 13 into the interior of the gas-tight protective housing 12 and introduced into the anode chambers of the fuel cell stack 11 in a fuel gas distributor 16 arranged on the anode input 15 on the bottom of the fuel cell stack 11 via a heat exchanger 14. The fuel gas flows through the anode chambers in essentially a vertical direction and emerges again on the anode output side 17 situated on the top of the fuel cell stack. The heat exchanger 14 is a gas/gas heat exchanger, which is traversed, on the one hand, by the fuel gas and, on the other hand, by a stream of cathode gas circulated within the gas-tight protective housing 12.
The cathode gas enters the fuel cell stack 11 at the cathode input 18 arranged laterally and leaves it at the cathode output 19 on the opposite side of the fuel cell stack. As can be deduced from FIG. 1, the flow directions of the cathode gas and fuel gas are perpendicular to each other. Maintenance of the gas streams in the protective housing 12 is accomplished by means of two fans 20, 21 arranged above the fuel cell stack 11, each of which are driven by electric motors 22, 23. A diffuser 24 and a static mixer 25 following it are arranged directly above the anode output 17 of the fuel cell stack 11. The anode waste gas leaving the anode output 17 is mixed with the cathode gas stream circulating in the housing 12 in the static mixer 25. Fresh air is also introduced to static mixer 24 via a line 26. Under the action of fans 20, 21 the gas mixture of anode waste gas, circulated cathode gas and fresh air is fed into a catalytic burner 27 arranged above the static mixer 25, in which combustible residual components of the anode waste gas are catalytically burned and converted to useful heat. The gas mixture leaving the catalytic burner, which now contains the main components of the cathode reaction with oxygen and carbon dioxide, is directed via fans 20, 21 to the cathode input 18, where it then flows through horizontally to the fuel cell stack 11. As mentioned above, after emergence at the cathode output 19, a partial stream of the cathode gas is fed back to the static mixer 24. A start heater 28 is preferably arranged in front of the cathode input 18, which brings the process gases to the operating temperature of about 600° C. during startup of the fuel cell arrangement 10. A diffuser 29 can also be arranged in front of the cathode input 18, which is supposed to permit homogeneous flow against the cell stack together with additional internals provided between fans 20, 21 and the cathode input 18. However, if, as in the depicted example, the heat exchanger 14 is also arranged in front of the cathode input 18, homogeneous flow against the cell stack can also be guaranteed by an appropriate configuration of the heat exchanger so that the additional diffuser 29 can optionally be dispensed with. Excess cathode exhaust leaves the fuel cell stack 11 via a cathode exhaust line 30 shown only schematically here.
The fuel cell arrangement described here is marketed by the applicant under the name HM 300 in a circular cylindrical protective housing.
In this known design principle the static mixer, the catalytic burner and the fans connected to them are directly arranged above the anode output of the fuel cell stack, which imposes high flow requirements on the circulation fan, namely both with respect to suction behavior of the fan in order to guarantee uniform mixing of fresh air, anode waste gas and cathode exhaust in the static mixer, and with respect to outflow behavior of the fan in order to guarantee uniform flow against the cell stack by the gas mixture. These requirements can be guaranteed in the previous design only by rectifiers and internals in the flow path, which, however, lead to pressure losses, which again requires higher fan power. In cell stacks with several hundred individual cells several fans arranged along the cell stack are also required in order to achieve homogeneous flow behavior.
A further drawback of the previous design is that the catalytic burner is arranged above the cell stack between the static mixer and fan. The catalyst during the operating time, however, is exposed to soiling, which can lead to a deterioration in flow and additional pressure losses so that the catalyst must regularly be cleaned. In the previous arrangement, however, the complete cell stack must be disassembled for this purpose, which is connected with a very high work cost and can only be conducted by the manufacturer.
Another drawback of the known design is that the mixer must be designed very compact directly above the anode output because of the limited space available so that satisfactory mixing can only be achieved by numerous internals with correspondingly high pressure loss. The manufacturing costs of the previously used mixer are therefore high.
Finally, the previous fuel cell arrangement permits only a few design degrees of freedom. The ratio of height and width of the fuel cell stack and the additional components arranged in the protective housing is essentially stipulated by the use of a circular cylinder protective housing and the degrees of freedom with respect to arrangement and dimensioning of the components arranged in the protective housing are limited. The layout of individual components specifically adapted to each other also means that numerous components must be newly designed, depending on the power layout of the system. The assembly cost of the previously used fuel cell arrangement is also high.